Europium activated yttrium, gadolinium borate ((Y, Gd)BO.sub.3 :Eu.sup.3+) is an efficient red emitting photo- and cathodo-luminescent phosphor. In recent years, efforts have been made to develop this phosphor, which is currently used in plasma display panels (PDP) due to its high quantum efficiency, persistence characteristics and reduced saturation.
These phosphors have been conventionally prepared by a high temperature (&gt;1200.degree. C.) solid state reaction (SSR) between Y.sub.2 O.sub.3 (Y source), Gd.sub.2 O.sub.3 (Gd source), Eu.sub.2 O.sub.3 (Eu source), boric acid and NH.sub.4 F or NH.sub.4 Cl (flux). The grain size of phosphor powders prepared by SSR is on the order 5 to 10 microns. Flat panel display devices such as PDPs, field emission displays (FED), and electro-luminescence (EL) panels, require thin fluorescent screens with fine grain (0.1 to 2 microns) phosphors for better performance and high efficiency. This requirement is more demanding in the case of PDPs, as the phosphors are screen printed between complicated structures, such as ribs. With small particles, it is possible to form a thin screen. Small particles also allow for a higher packing density and less binder content.
Originally, phosphors having a small particle size were obtained by grinding, crushing or milling large phosphor particles. Phosphors obtained by these methods showed greatly reduced efficiency, with little or no control over the particle morphology. More recently, "no mill" phosphors have been prepared by rapid cooling of a phosphor mass after completion of the SSR and with either a short-time firing at a high temperature, or a longer duration firing at a lower temperature. These processes help minimize further growth of phosphor crystals. In the presence of flux or inhibitors, particle size distribution (PSD) and morphology of the phosphor can be controlled. See M. Kotaisamy, R. Jagannthan, R. P. Rao, M. Avudaithai, L. K. Srinivassan and V. S. Sundaram, J. Electrochem Soc. 142 (1995) 3205. It has been proposed that sub-micron size phosphor particles can be synthesized by a sol-gel process. See R. P. Rao, J. Electrochem Soc. 143 (1996) 189. Small phosphor particles have been synthesized by hydrothermal methods. See T. R. N. Kutty, R Jagannthan, R. P. Rao, Mater. Res. Bull. 25 (1990) 1355.
Most past work on red phosphors has been related to Eu.sup.3+ activated yttrium oxide for fluorescent lamps and yttrium oxy-sulfide for cathode ray tubes at(CRTs). Since neither of these phosphors are suitable for alternating current PCPs or (AC PDPs), attempts have been made to develop new phosphors useful in PDPs that are excitable at wavelengths generated by Xenon gas (147 and 173 nm). Eu.sup.3+ activated borates of yttrium and lanthanum series are proposed in the literature, however, limited information is available on the preparation and luminescence of europium activated yttrium gadolinium borate phosphors. U.S. Pat. No. 4,202,794 to Lehmann proposes an improved phosphor composition expressed by the general formulation xCaO.y(Y+Eu).sub.2.zB.sub.2 O.sub.3 wherein x is from 32 to 38, y is from 31 to 40 and z is from 25 to 31. Such phosphors are alleged to provide a higher photoluminescence efficiency compared to a similar type of phosphor, when excited by 254 nm radiation and emitting in the red region of the visible spectrum.
Since past synthesis methods involved high temperature solid state reactions, the control over the impurity concentration, PSD and morphology was limited. Also, it has been found that the phosphor screens formed with small particles (0.5 to 2.0 microns) exhibit improved performance. This is particularly true for PDPs. However, most of the above methods fail to provide small particles (0.1 to 2.0 microns). The growth of small particles can be possible in case of sol-gel or solution methods where the reaction temperatures are well below the normal solid state temperatures.